This invention relates to an apparatus for forming ribs with sideward projections, preferably T-shaped ribs, in a plastic pipe with annular ribs. The invention also relates to a method for forming ribs with sideward projections, preferably T-shaped ribs, in a plastic pipe with annular ribs. The invention further relates to a pipe having adjacent annular ribs in its outer surface, the areas between the ribs forming grooves for sealing rings arranged at socket joints for sealing the space between the pipe and the socket.
Plastic ribbed pipes are known from the prior art. The provision of ribs on the outer surface of pipes increases the cost of manufacture, but this kind of pipes are nevertheless used very widely as they have many good properties.
Ribs are usually rectangular in cross-section, or they taper slightly from the pipe wall towards their outer edge. U.S. Pat. Nos. 4,566,496 and 4,753,833, however, disclose ribs having a T-shaped cross-section achieved by forming a projection at the outer edges of the ribs on both sides. This shape of the rib increases the rigidity of the pipe against radial compression of the pipe; alternatively, the same rigidity is achieved with less material, which reduces the costs.
However, plastic pipes with T-shaped ribs are not produced on any larger scale, which is mainly due to problems associated with the manufacturing technique. Plastic pipes with annular ribs are produced in so-called corrugators where molten plastic material is fed through a nozzle to form a layer around a mandrel, and the plastic material is surrounded within the area of the mandrel by moving chill moulds with grooves in their inner surfaces for forming the ribs. In order that the chill moulds could be detached from the pipe after moulding the pipe, the sides of the ribs have to be straight. For this reason, plastic pipes with T-shaped ribs have previously been produced from plastic strips having one or more T-shaped ribs on one side. The strips are wound around the mandrel into a spiral, so that a solid plastic pipe is obtained when the strips are seamed. A plastic pipe produced in this way is expensive. It is also to be noted that the ribs on its outer side are not annular but run spirally along the outer surface of the pipe. This is a disadvantage as sealing rings cannot be fitted in the grooves between the ribs at socket joints, or the use of such spiral grooves for sealing purposes is very difficult.